1. Field of the Invention
The invention concerns a method for making an alternation of layers of monocrystalline semiconducting material and layers of insulating material or insulator and, notably, a method for making at least one thin layer of a semiconducting material on a layer of insulating material which is itself deposited on a monocrystalline semiconducting substrate, said substrate being a semiconductor which may or may not be different from that of the thin layer so as to obtain a hetero-epitaxial structure.
More particularly, the invention relates to the field of thin layers of a monocrystalline material, epitaxially grown by chemical vapor deposition (CVD) or vapor phase deposition (VPE), on a substrate with a possibly different nature. The method described according to the invention enables, in particular, the removal of two-dimensional extended faults (twins, stacking faults, antiphase walls in ordered alloys, sub-grain boundaries etc.) as well as linear faults or dislocation lines.
The method shall be described with reference to examples of homo-epitaxy or hetero-epitaxy of semiconducting materials. However, it is quite clear that this method is more general and that its application is not restricted to semiconducting materials alone.
2. Description of the Prior Art
The origin of the extended faults found in thin layers, epitaxially grown by a chemical vapor deposition on a monocrystalline substrate with a different nature and lattice parameter lies essentially in the initial mode of nucleation of the deposit.
Schematically, for chemical vapor deposition, we observe three modes of nucleation and growth of a monocrystalline thin film on an also monocrystalline substrate.
First of all, a layer-by-layer mode of nucleation and growth where the arrangement of the deposited atoms takes place in the exposed atomic plane of the substrate (two-dimensional arrangement), for example by lateral growth at the level of the monolayer from mono-atomic steps which form preferred adsorption sites. When all of the sites of the deposits are saturated on the exposed surface of the substrate, the growth continues at the level of the second monolayer according to the same mechanism and so on, plane by plane. This type of mechanism is observed in cases of homo-epitaxy of course (silicon doped on silicon for example) but also in the case of hetero-epitaxy of metals (Cd or W for example) or semiconductors (Ga.sub.1-x Al.sub.x As on GaAs, GaInAsP on InP . . . ). The extended faults generated during the epitaxy are dislocations of interfaces which release the elastic energy stored in the deposit owing to a possible mismatching of its lattice parameter with that of the substrate. These dislocations get spread in the thickness of the deposit and their total elimination is practically impossible.
The second mode of growth results from a mechanism of nucleation in islands: according to this mechanism, clusters of atoms are formed initially on the surface of the substrate, generally on sites of emerging faults. Those of these clusters which achieve the critical seed size then grow to form condensed face islands. These islands coalesce when they come together in order to form a film which becomes truly continuous only after it has reached a certain thickness. This type of growth is observed when the bonding energy between atoms of the deposit is greater than the bonding energy between atoms of the deposit on the one hand and atoms of the substrate on the other. This is often the case with films of metals or semiconductors on insulating substrates. The faults present in these films consist, firstly, of lines of dislocations (just as above) which enable the mechanical energy due to the mismatching of the lattice to be released and, secondly, of twins which, because of the particular growth mechanism, are generated at the junction between islands in order to adapt to slight disorientations. These twins grow with the film and are, consequently, very difficult to remove. Thus, more than 25 years after the first published work on silicon hetero-epitaxy on a sapphire substrate, we still have to be content with a highly twinned epitaxiated material, and the technology for making the devices from the material in question has to adapt to its poor crystalline quality. In particular, it is impossible to use ionic implantation because, during thermal annealing, the point faults of implantation precipitate on the extended faults and do not heal.
Finally, the third mode of growth observed is a combination of the first two modes described above, i.e. the growth starts in two dimensions (the first example described above) on several monolayers, then islands start appearing according to the above-described second mechanism. In coalescing, these islands will then give birth to a continuous film. Just as in the previous cases, the faults will consist of dislocation lines and twins. The removal of these faults, which can be done only by annihilation (the meeting of two dislocations of opposite Burgers vectors), will be very difficult if it is sought to remain in a reasonable range of deposited thickness. The situation is described in FIG. 1 where the epitaxiated CME thin layer deposited on the substrate 1 has dislocations or else twin planes shown in dashes.
In addition to the other above described faults inherent to the growth mode, there are faults generated by the fact that the deposit is put under stress when cooling, for the expansion coefficients of the substrate and of the deposit are only very rarely matched. This cooling process can generate other dislocations as well as slippage phenomenona leading to the formation of stacking faults.
The method according to the invention makes it possible to stop the propagation of most of the above described faults and, consequently, to obtain thin layers of practically perfect crystalline quality. The method according to the invention can also be used to obtain stacks of monocrystalline layers insulated by amorphous or polycrystalline dielectric material.